The rapidly growing experimental literature on social pressure is not only of immense significance in political science, where problems of collective participation abound, it also speaks to cognate literatures in social psychology and behavioral economics. Recent years have seen a surge of interest in compliance with pro-social requests. For example, field experiments have gauged the extent to which different kinds of feedback encourage consumers to conserve resources (Goldstein et al. 2008; Ayres et al. 2009) or make charitable donations (Karlan and McConnell 2009). Many of these interventions, particularly those indicating that compliance will be monitored and disclosed to others, are similar in structure to the voting experiments described above. Given the pace, scale, and nuance of social pressure research across social science disciplines, we should soon expect a new empirically grounded understanding of the conditions under which people comply with norms.